DESCRIPTION: Persons who walk with lower-limb prostheses are generally less efficient ambulators than able-bodied individuals (Waters et al., 1976) and their stability is compromised, attributable in part t deficiencies in the function of their prostheses (Gard & Fatone, 2004). Anatomical ankle joint stiffness in able-bodied persons adapts with walking speed (Hansen et al., 2004) and for standing (Hansen & Wang, 2010). Fitting lower-limb amputees with prosthetic foot and ankle mechanisms that attempt to replicate corresponding anatomical functions is desirable (Hansen et al., 2004a,b, 2007, 2010). We previously demonstrated that prosthetic ankle joints improve walking performance in persons with transtibial (below-knee) amputation (Su et al., 2008, 2009, 2010). In that study, research subjects clearly preferred walking with the prosthetic ankle components, but several indicated that they felt unstable during standing (Su et al., 2010). Subsequent analyses of those data indicated that the addition of a compliant prosthetic ankle unit significantly reduced the radius of the ankle-foot roll-over shape (Gard et al., 2011), which can adversely affect standing stability and gait performance (Gard & Childress, 2001; Klodd et al., 2010a,b). The purpose of this investigation is to determine how systematically varying the prosthetic foot keel stiffness and prosthetic ankle joint stiffness affects standing and walking in persons with unilateral, transtibial amputations. The specific aims for this study are: 1. To determine how different combinations of prosthetic foot and ankle stiffness affect gait biomechanics of unilateral, transtibial prosthesis users. Kinematic, kinetic and energy expenditure data will be collected as subjects walk at different speeds and with different combinations of prosthetic foot and ankle stiffness. 2. To determine how different combinations of prosthetic foot and ankle stiffness affect standing stability of unilateral, transtibial prostheis users. Standing balance of subjects will be evaluated using a series of tests that measure balance and recovery stability as balance is perturbed. Subjects will also be administered questionnaires to document their perceptions of comfort, exertion and stability while using the different prosthetic foot-ankle configurations. Compliant foot-ankle mechanisms that allow for a normal range of ankle joint motion during walking are expected to increase gait performance, but decrease standing stability. Conversely, a rigid foot-ankle combination will likely maximize standing stability, but decrease gait performance. Determination of an optimal prosthetic foot and ankle stiffness combination will require a compromise between the apparent disparate objectives for these two activities. Increased understanding about how different prosthetic foot-ankle stiffness combinations affect standing and walking abilities will facilitate appropriate component selection by prosthetists, encourage development of prosthetic foot-ankle mechanisms with adaptable stiffness, and ultimately improve quality of life for prosthesis users.